Abstract
The 40 km resolution SMOS (Soil Moisture and Ocean Salinity) soil moisture, previously disaggregated at a 1 km resolution using the DISPATCH (DISaggregation based on Physical And Theoretical scale CHange) method based on MODIS optical/thermal data, is further disaggregated to 100 m resolution using Sentinel-1 backscattering coefficient (σ°). For this purpose, three distinct radar-based disaggregation methods are tested by linking the spatio-temporal variability of σ° and soil moisture data at the 1 km and 100 m resolution. The three methods are: (1) the weight method, which estimates soil moisture at 100 m resolution at a certain time as a function of σ° ratio (100 m to 1 km resolution) and the 1 km DISPATCH products of the same time; (2) the regression method which estimates soil moisture as a function of σ° where the regression parameters (e.g., intercept and slope) vary in space and time; and (3) the Cumulative Distribution Function (CDF) method, which estimates 100 m resolution soil moisture from the cumulative probability of 100 m resolution backscatter and the maximum to minimum 1 km resolution (DISPATCH) soil moisture difference. In each case, disaggregation results are evaluated against in situ measurements collected between 1 January 2016 and 11 October 2016 over a bare soil site in central Morocco. The determination coefficient (R2) between 1 km resolution DISPATCH and localized in situ soil moisture is 0.31. The regression and CDF methods have marginal effect on improving the DISPATCH accuracy at the station scale with a R2 between remotely sensed and in situ soil moisture of 0.29 and 0.34, respectively. By contrast, the weight method significantly improves the correlation between remotely sensed and in situ soil moisture with a R2 of 0.52. Likewise, the soil moisture estimates show low root mean square difference with in situ measurements (RMSD= 0.032 m3 m−3).
Highlights
IntroductionSoil moisture (sm) is a land state variable governing the interaction between the land surface and atmosphere through playing its role in various components of the water and energy cycle, such as evapotranspiration, groundwater recharge, and surface runoff
Soil moisture is a land state variable governing the interaction between the land surface and atmosphere through playing its role in various components of the water and energy cycle, such as evapotranspiration, groundwater recharge, and surface runoff
The DISPATCH product is obtained from the daily global maps of soil moisture, land surface temperature and vegetation index and its associated parameters over a three-day moving window [29] while the in situ measurements are obtained from the instantaneous measurements of the dielectric constant
Summary
Soil moisture (sm) is a land state variable governing the interaction between the land surface and atmosphere through playing its role in various components of the water and energy cycle, such as evapotranspiration, groundwater recharge, and surface runoff. Comprehensive monitoring programs providing high quality soil moisture information are needed for precisely modeling the exchange of water, energy, and carbon fluxes between land and atmosphere. The high sensitivity of microwave imagery to soil moisture and its ability to work under all weather conditions make it a suitable candidate for capturing the spatial and temporal variability of soil moisture [2]. The Soil Moisture and Ocean Salinity (SMOS) mission, launched in November 2009, utilizes an L-band interferometric radiometer for providing topsoil (3–5 cm) moisture maps at 30–55 km resolution [5,6]. SMOS soil moisture products have been regularly validated [7,8] and evaluated as appropriate for hydro-climate applications [9], higher spatial resolutions detailing the soil moisture distribution within the passive microwave pixel is highly required, especially for agricultural applications [10]
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